Sandwich arc structure for preventing metal to contact from shifting

ABSTRACT

A sandwich ARC structure for preventing metal to contact from shifting, the sandwich ARC structure comprising a first Ti layer formed on a metal laer and a first TiN layer formed on the first Ti layer. A second Ti layer is formed on the first TiN layer and a second TiN layer is formed on the second Ti layer. Wherein the sandwich ARC structure formed of first Ti/first TiN/second Ti/second TiN will reduces the tress between said metal layer and a dielectric layer formed below the metal layer.

FIELD OF THE INVENTION

[0001] The present invention relates generally to a semiconductor technology, and more specifically, to a sandwich anti-reflection coating (ARC) for preventing metal to contact from shifting.

BACKGROUND OF THE INVENTION

[0002] Flash memory is one of the segments of nonvolatile memory devices. The device includes a floating gate to store charges and an element for electrically placing the charges on and removing the charges from the floating gate. One of the applications of flash memory is BIOS for computer. Typically, the high-density nonvolatile memories can be applied for the mass storage of portable handy terminals, solid state camera and PC cards. That is because that the nonvolatile memories exhibit many advantages, such as fast access time, low power dissipation, and robustness. Bergemont proposed another cell array for portable computing and telecommunications applications, which can be seen in Bergmont et al., “Low Voltage NVG™: A New High Performance 3 V/5 V Flash Technology for Portable Computing and Telecommunications Applications”, IEEE Trans. Electron. Devices, vol. ED-43, p. 1510, 1996. This cell structure is introduced for low voltage NOR virtual ground (NVG) flash memory having fast access time. In the flash array schematic, field oxides (FOX) are formed between cells such that a poly extension on the FOX of each cell provides adequate gate coupling ratio (GCR). In the art, the access time is one of the key concerns for low voltage read operation. The NVG array uses select devices to achieve a fast access time by reducing the pre-charge time to that of a single segment rather than the full bit-line.

[0003] Various nonvolatile memories have been disclosed in prior arts. For example, Mitchellx has proposed EPROMs with self-aligned planar array cell. In this technique, buried diffusion self-aligned to the floating gate avalanche injection MOS transistor are used for the bit lines. Cross point array technology has been disclosed. The self-aligned source and drain will allow this device to be optimized even further for programming speed. See A. T. Mitchellx, “A New Self-Aligned Planar Cell for Ultra High Density EPROMs”, IEDM, Tech. pp. 548-553, 1987”. The formation of nonvolatile memories toward to the trends of low supply power and fast access, because these requirements are necessary for the application of mobile computing systems. One important key parameter of high performance memory is capacitive-coupling ratio. Flash memory needs the charges to be hold in the floating gate for a long period of time. Therefore, the dielectric that is used for insulating the floating gate needs to be high performance. At present, the low voltage flash memory is applied with a voltage of about 3V or 5V during charging or discharging the floating gate. As known in the art, tunneling is a basic technology in charging or discharging. In order to obtain high tunneling efficiency, the thickness of the dielectric between the floating gate and substrate have to be scaled down due to the reduced supply voltage. However, it will degrade the reliability of the dielectric when the thickness of the dielectric is scaled down to below 10 nm.

[0004] The metal to contact alignment is also an important parameter for the manufacture of flash memory. The shift between the metal and contact can be observed by SEM and AEI REG data. Please refer to FIG. 1, it shows the top view of a wafer 10 having a plurality of sub-area defined thereon. The device or circuits will be formed on the wafer. Please also refer to FIG. 2a to FIG. 2f, a metal layer 26 is formed over a dielectric layer 20 with a contact hole 22 filled with conductive material 24. The profile of the metal layer 26 will be changed by the stress between the metal 26 and the dielectric layer 20. Subsequently, a photoresist for patterning pattern will be formed over the metal layer 6. However, the lithography uses the metal profile for the alignment during the lithography procedure. The shifted metal profile provides incorrect information, thereby causing the photoresist pattern to shift from the predetermined location as shown in FIG. 2e. After the etching of the metal layer, the metal pattern will not on the correct position as cna be seen in FIG. 2f. Referencing the FIG. 1, from the SEM of the left and right side area, the contact is almost exposed by the metal pattern, it means that the shift caused by the stress between thereof is very serious. The issue will degrade the performance of the device, therefore what is required is a method for solving the problem.

SUMMARY OF THE INVENTION

[0005] A sandwich ARC structure for preventing metal to contact from shifting, the sandwich ARC structure comprising a first Ti layer formed on a metal layer and a first TiN layer formed on the first Ti layer. A second Ti layer is formed on the first TiN layer and a second TiN layer is formed on the second Ti layer. Wherein the sandwich ARC structure formed of first Ti/first TiN/second Ti/second TiN will reduces the tress between said metal layer and a dielectric layer formed below the metal layer. The thickness of the first Ti layer is about 10 to 200 angstroms. Wherein the thickness of the first TiN layer is about 10 to 300 angstroms. The thickness of the second Ti layer is about 10 to 200 angstroms. The thickness of the second TiN layer is about 10 to 300 angstroms.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] The foregoing aspects and many of the attendant advantages of this invention will become more readily appreciated as the same becomes better understood by reference to the following detailed description, when taken in conjunction with the accompanying drawings, wherein:

[0007]FIG. 1 is a top view of a semiconductor wafer illustrating the metal to contact overlay shift issue at the side area of the wafer according to the prior art;

[0008]FIGS. 2a-2 f are cross-sectional views of a semiconductor wafer illustrating the metal to contact overlay shift issue at the side area of the wafer according to the prior art; and

[0009]FIG. 3 is a top view of a semiconductor wafer illustrating the metal to contact overlay shift issue solved by the sandwich ARC structure according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention proposes a novel method to fabricate a semiconductor device includes but not limited to flash nonvolatile memory. In the sandwich ARC structure, Ti/TiN/Ti/TiN will be introduced. The typical shift between the metal to contact will be eliminated by the present invention. The detail description will be seen as follows.

[0011] A semiconductor substrate is provided for the present invention. In a preferred embodiment, a single crystal silicon substrate with a <100> crystallographic orientation is provided. A plurality of isolations are formed on the substrate. In general, field oxide (FOX) isolation or trench isolation techniques can be introduced to serve as the isolations. For example, the FOX regions can be formed via lithography and etching steps to etch a silicon nitride/silicon dioxide composition layer. After the photoresist is removed and wet cleaned, thermal oxidation in steam environment is used to grow the FOX regions. A gate, poly and ILD patterns are form sequentially for array cell. After contact photo and etch process, W deposition and W CMP is process for W plug. Al deposition is form before the ARC film deposition which both act as metal film. After the formation of metal line pattern, it is found that the metal line is not correctly overlapped to contact layer although the overlay data has been compensated by overlay mark at metal photo process. From the experience, the result shows that the Aluminum deposition temperature influence is small. At the range of plus/minus 100 degree, the overlay shift issue can still be observed. While skip the ARC film atop on the Al film, the metal to contact has better overlay performance without center-edge difference. The overlay shift is raised by the ARC film stress that is mainly due to the TiN thin film.

[0012] The sandwich ARC structure is provided for reducing the shift from original 100 nm to less than 25 nm. Please turning to FIG. 3, it shows the top view of the wafer observed from the left, central and right area, respectively. Apparently, the present invention sandwich ARC structure improves the shift problem. Namely, the metal to contact shift is reduced. The sandwich structure includes a lower Ti layer formed and a lower TiN layer having a thickness of about 10-300 angstroms. The thickness of the lower Ti layer is about 10-200 angstroms. An upper Ti layer having a thickness of about 10-200 angstroms is formed on the lower TiN layer. An upper TiN layer is formed on the Ti layer. The thickness is preferably 10-300 angstroms. The overlay shift enhancement mechanism could result from the ARC film stress that is mainly due to the TiN film. The improvement method according t the present invention is to introduce the sandwich structure including Ti/TiN/Ti/TiN.

[0013] As will be understood by persons skilled in the art, the foregoing preferred embodiment of the present invention is illustrative of the present invention rather than limiting the present invention. Having described the invention in connection with a preferred embodiment, modification will now suggest itself to those skilled in the art. Thus, the invention is not to be limited to this embodiment, but rather the invention is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims, the scope of which should be accorded the broadest interpretation so as to encompass all such modifications and similar structures. 

What is claimed is:
 1. A sandwich ARC structure for preventing metal to contact from shifting, said sandwich ARC structure comprising: a first Ti layer formed on a metal layer; a first TiN layer formed on said first Ti layer; at least a second Ti layer formed on said first TiN layer; and at least a second TiN layer formed on said second Ti layer; wherein said sandwich ARC structure formed of first Ti/first TiN/second Ti/second TiN will reduces the tress between said metal layer and a dielectric layer formed below said metal layer.
 2. The structure of claim 1, wherein the thickness of said first Ti layer is about 10 to 200 angstroms.
 3. The structure of claim 1, wherein the thickness of said first TiN layer is about 10 to 300 angstroms.
 4. The structure of claim 1, wherein the thickness of said second Ti layer is about 10 to 200 angstroms.
 5. The structure of claim 1, wherein the thickness of said second TiN layer is about 10 to 300 angstroms.
 6. A sandwich ARC structure for preventing metal to contact from shifting, said sandwich ARC structure comprising: a first Ti layer having a thickness of about 10 to 200 angstroms formed on a metal layer; a first TiN layer having a thickness of about 10 to 300 angstroms formed on said first Ti layer; a second Ti layer having a thickness of about 10 to 200 angstroms formed on said first TiN layer; and a second TiN layer having a thickness of about 10 to 300 angstroms formed on said second Ti layer; wherein said sandwich ARC structure formed of first Ti/first TiN/second Ti/second TiN will reduces the tress between said metal layer and a dielectric layer formed below said metal layer. 